1. Field of the Invention
The present invention relates to a rotation driving device, and more particularly to a rotation driving device suitable for use in a diaphragm device of an optical apparatus, such as a camera, which employs a stepping motor as a drive source.
2. Description of Related Art
In a conventional rotation driving device using a stepping motor, for example, in a diaphragm device used in an optical apparatus or the like, a ring-like rotary member (rotary ring) and diaphragm blades are interposed between two ring-like fixed members (first and second fixed members), thereby constituting a diaphragm unit. Dowels provided on the diaphragm blades are fitted into and engaged with a plurality of cams formed in the first fixed member (cam plate) and holes bored in the rotary ring, respectively. Further, the drive of the diaphragm blades are effected by the sliding movements of the dowels along the cams according to the rotation of the rotary ring, so that the diaphragm blades are driven to cover aperture holes formed in the first fixed member and the rotary ring and open the aperture holes up to a set aperture value.
In such an arrangement, the rotational fitting of the rotary ring and the first fixed member is required to be made at the outer circumference of the rotary ring. This is because the diaphragm blades are placed in between the rotary ring and the first fixed member and are arranged to pass on the inner circumferential side of the rotary ring when being driven and, hence, it is impossible that the rotational fitting of the rotary ring and the first fixed member is made on the inner circumferential side of the rotary ring.
Meanwhile, in the case of the above-mentioned conventional example, since the rotary ring is so configured as to be rotationally fitted in the first fixed member at the outer circumference of the rotary ring, it is said that the rotational fitting is made at a larger outer diameter portion, thereby creating a problem in that the rotational load torque to the rotary ring would become large. In other words, the rotational fitting at the larger outer diameter portion brings about a larger rotational load torque to the rotary ring, even if a frictional force at the fitting portion is small.
On the other hand, while it is conceivable to employ an electromagnetic motor, such as a stepping motor, as a drive source for the rotary ring, in the case of using the stepping motor, the large rotational load torque as mentioned above causes a disadvantage in that the deterioration of the rotational performance and rotational efficiency would take place and, in the worst situation, the loss of synchronism is brought about to stop the stepping motor.
In addition, in the field of cameras or the like, the recent trend is toward making it compact, and, therefore, the size reduction of the stepping motor itself has been in demand. Since the size reduction of the stepping motor naturally leads the motor itself to lowering its power, the deterioration of the aforesaid rotational performance, i.e., the rotational efficiency, becomes remarkable, and, further, the loss of synchronism tends to more easily take place.
One possible solution of this problem is that, instead of the rotational fitting of the rotary ring and the first fixed member, the rotational fitting of the rotary ring is made with the second fixed member (ring-like base plate), which finally covers the diaphragm blades and the rotary ring to put them therein in a state where the diaphragm blades and the rotary ring are incorporated onto the first fixed member. In this arrangement, the rotational fitting at a small diameter portion can be attained. The reason is that the diaphragm blades are absent between the rotary ring and the second fixed member (ring-like base plate) in their thrust directions and, hence, the diameter for the rotational fitting can be defined at a closest portion to the inner diameter of the aperture hole through which a photographing light beam passes.
According to this structure, there is no fitting between the rotary ring and the first fixed member at the large outer diameter portion. Therefore, the increase in the rotational load torque can be prevented by the rotational fitting at the small diameter portion.
However, in the case of the above solution method, considering the assembling procedure for the fitting between the rotary ring and the ring-like base plate, the incorporation of the diaphragm blades is first made in such a manner that one dowel of each of the diaphragm blades is fitted in the cam formed on the first fixed member and, subsequently, the incorporation of the rotary ring is made in such a manner that the other dowel of each of the diaphragm blades is fitted in the hole portion of the rotary ring, and, finally, the ring-like base plate is positioned and fixed.
In the above assembling procedure, the rotary ring with the diaphragm blades set on the first fixed member is required to be fitted in the ring-like base plate under the condition that there is no fitting between the rotary ring and the first fixed member, i.e., under the condition that no centering of the rotary ring takes place (or after the centering of the rotary ring is taken). Therefore, the ring-like base plate needs to be fitted on the rotary ring while being positioned and supported with respect to the first fixed member, so that the assembling efficiency is very much lowered, taking a long assembling time and bringing up the manufacturing cost.